Young infants (less than 8 months of age) have decreased ELISA F and G and neutralizing antibody responses to respiratory syncytial virus (RSV) infection. Age primarily affects the response to the F glycoprotein and pre-existing antibody affects the response to the G glycoprotein. Infants and children less than two years of age primarily see the heavily glycosolated G glycoprotein as a protein antigen, i.e., they mount an ELISA IgG1 and IgG3 subclass response to the G glycoprotein. Adults, however, generate an IgG2 response equivalent to their IgG1 response. Thus, the G protein can be seen by the immune system as a protein and/or carbohydrate antigen. Infants and childen immunized with formalin-inactivated RSV vaccine developed titers of ELISA antibodies equivalent to those of individuals infected with RSV, but the neutralizing antibody titers of the vaccinees were low. Thus, treatment of RSV with formalin appears to have altered the epitopes of the F and/or G glycoproteins that stimulate neutralizing antibodies with the result that the immune response to vaccine consisted largely of "nonfunctional" (i.e., non-neutralizing) antibodies. In tests employing post-infection sera of young infants the Long and 18537 strains of RSV were shown to differ by approximately 3 fold in cross neutralization assays. The G glycoproteins were about 8-fold different by ELISA and the F proteins about 2-fold different. Both the HN and F glycoproteins of a parainfluenza type 2 virus (SV-5) (in a Vaccinia-SV-5 recombinant) induced SV-5 neutralizing antibodies and resistance to SV-5 virus challenge.